Larry J. Hornbeck

hbek@dlep1.itg.ti.com

Texas Instruments
Digital Video Operations
Dallas, Texas 75265

ABSTRACT

Electronic projection display technology for high-brightness applications
had its origins in the Gretag Eidophor, an oil film-based projection system
developed in the early 1940s. A number of solid state technologies have challenged
the Eidophor, including CRT-addressed LCD light valves and active-matrix-addressed
LCD panels. More recently, in response to various limitations of the LCD technologies,high-brightness
systems have been developed based on Digital light Processing tm
technology. At the heart of the DLP tm projection display is the
Digital Micromirror Device tm (DMD)tm, a semiconductor-based
array of fast, reflective digital light switches that precisely control a light
source using a binary pulse width modulation technique.

This paper describes the design, operation, performance, and advantages of
DLP-based projection systems for high-brightness,high-resolution applications.
It also presents the current status of high-brightness products that will soon
be on the market.

A survey of
high-brightness (>1000 lumens) electronic projection displays
is shown in Figure 1.
The brightness (lumens) is plotted against the brightness efficiency (lumens/watt).Three
types of projection display technologies are compared in Figure 1, oil film,
CRT-LCD, and AM-LCD. Developed in the early 1940s at the Swiss Federal Institute
of Technology and later at Gretag AG, oil film projectors (including the GETalaria)
have been the workhorse for applications that require projection displays of
the highest brightness [1]. But the oil film projector has a number of limitations
including size,weight, power, setup time, stability, and maintenance. In response
to these limitations, LCD-based technologies have challenged the oil film projector.
These LCD-based projectors are of two general types: (1) CRT-addressed LCD lightvalves
and (2) active-matrix (AM) LCD panels.

LCD-based projectors have not provided the perfect solution for the entire
range of high-brightness applications. CRT-addressed LCD light valves have setup
time and stability limitations. Most active-matrix LCDs used for high-brightness
applications are transmissive and, because of this, heat generated by light
absorption cannot be dissipated with a heat sink attached to the substrate.
This limitation is mitigated by the use of large-area LCD panels with forced-air
cooling. However, it may still be difficult to implement effective cooling at
the highest brightness levels.

In response to these and other limitations,
as well as to provide superior image quality under the most demanding environmental
conditions, high-brightness projection display systems have been developed based
on Digital Light Processing tm technology. DLP tm is based
on a micro-electromechanical system (MEMS) device known as the Digital Micromirror
Device tm (DMD)tm. The DMD, invented in 1987 at Texas
Instruments, is a semiconductor-based array of fast, reflective digital light
switches that precisely control a light source using a binary pulse width modulation
technique. It can be combined with image processing, memory, a light source,
and optics to form a DLP system (Figure 2) capable of projecting large,
bright, seamless, high-contrast color images.

Figure 3 shows
a DLP projector in an auditorium environment. This photo was taken at the Texas
instruments Digital Imaging Business Center in Dallas, Texas.

DLP-based projection displays are well-suited to high-brightness and high-resolution
applications: (a) the digital light switch is reflective and has a high fill
factor, resulting in high optical efficiency at the pixel level and low pixelation
effects in the projected image; (b) as the resolution and size of the DMD increase,
the overall system optical efficiency grows because of higher lamp-coupling
efficiency; (c) because the DMD operates with conventional CMOS voltage levels
(~5volts), integrated row and column drivers are readily employed to minimize
the complexity and cost impact of scaling to higher resolutions; (d) because
the DMD is a reflective technology,the DMD chip can be effectively cooled through
the chip substrate, thus facilitating the use of high-power projection lamps
without thermal degradation of the DMD; and(e) finally, DLP-based systems are
all-digital (digital video in, digital light out), so reproduction of the original
video source material is accurate and the image quality is stable with time
[2].

The general movement of the display industry is in the digital direction.
Digital sources that are currently available include digital video disk (DVD),
digital satellite system(DSS), and the Internet (World Wide Web). In the future,
the recently approved Advanced Television Standard (ATV) and the digital distribution
of movies (digital cinema) will be added to the list of digital sources. Interfacing
these digital sources to currently available analog displays requires digital-to-analog
conversion and, in some instances, analog encoding (e.g., s-video or composite),
which result in degradation of the source image quality. DLP-based displays,
on the other hand, preserve the digital integrity of the source image all the
way to the eye. The result is the best possible video quality.

A comprehensive, chronological list of DLP and DMD publications and presentations
[2-59] is presented in Section 8.0.The list includes general DLP review papers
and papers on early DMD development, system electronics, optics, DMD mechanical
modeling, manufacturing, and reliability. It also includes references to DMD-based
digital printing technology [4,24,37,56].

The DMD light switch (Figure 4) is a member of a class of devices known
as micro-electromechanical systems.

Other MEMS devices include pressure sensors, accelerometers, and microactuators.
The DMD is monolithically fabricated by CMOS-like processes over a CMOS memory.
Each light switch has an aluminum mirror, 16 um square, that can reflect light
in one of two directions, depending on the state of the underlying memory cell.
Rotation of the mirror is accomplished through electrostatic attraction produced
by voltage differences developed between the mirror and the underlying memory
cell. With the memory cell in the on (1) state, the mirror rotates to +10 degrees.

With the memory cell in the off (0) state, the mirror rotates to -10 degrees.
A close-up of DMD mirrors operating in a scanning electron microscope (SEM)
is shown in Figure 5.

By combining the DMD with a suitable light source and projection optics (Figure
6), the mirror reflects incident light either into or out of the pupil of
the projection lens by a simple beam-steering technique.

Thus, the (1) state of the mirror appears bright and the (0) state of the mirror
appears dark. Compared to diffraction-based light switches, the beam-steering
action of the DMD light switch provides a superior tradeoff between contrast
ratio and the overall brightness efficiency of the system.

Grayscale is achieved by binary pulse width modulation of the incident light.
Color is achieved by using color filters, either stationary or rotating, in
combination with one, two, or three DMD chips (Section 3.2).

The DMD light switch is able to turn light on and off rapidly by the beam-steering
action of the mirror. As the mirror rotates,it either reflects light into or
out of the pupil of the projection lens, to create a burst of digital light
pulses that the eye interprets as an analog image (Figure 2). The optical
switching time for the DMD light switch is ~2 us. The mechanical switching time,
including the time for the mirror to settle and latch, is ~15 us [36].

The technique for producing the sensation of grayscale to the observer's eye
is called binary pulse width modulation. The DMD accepts electrical words representing
gray levels of brightness at its input and outputs optical words, which are
interpreted by the eye of the observer as analog brightness levels. The details
of the binary pulsewidth modulation (PWM) technique are illustrated in Figure
7.

For simplicity, the PWM technique is illustrated for a 4-bit word (2 4
or 16 gray levels). Each bit in the word represents a time duration for light
to be on or off (1 or 0). The time durations have relative values of 20,21,
22, 23, or 1, 2, 4, 8. The shortest interval (1) is called
the least significant bit (LSB). The longest interval (8) is called the most
significant bit (MSB). The video field time is divided into four time durations
of 1/15, 2/15, 4/15, and 8/15 of the video field time. The possible gray levels
produced by all combinations of bits in the 4-bit word are 24 or
16 equally spaced gray levels (0, 1/15, 2/15 . . . 15/15). Current DLP systems
are either 24-bit color (8 bits or 256 gray levels per primary color) or 30-bit
color (10 bits or 1024 gray levels per primary color). In the simple example
shown in Figure 7, spatial and temporal artifacts can be produced because
of imperfect integration of the pulsed light by the viewer's eye.

These artifacts
can be reduced to negligible levels by "bit-splitting" technique
[26]. In this technique, the longer duration bits are subdivided into shorter
durations, and these split bits are distributed through-out the video field
time. DLP displays combine pulsewidth modulation and bit-splitting to produce
"true-analog" sensation, but with greater accuracy and stability than can
be achieved by analog projection systems.

An organic sacrificial layer is removed by plasma etching to produce air gaps
between the metal layers of the superstructure. The air gaps free the structure
to rotate about two compliant torsion hinges. The mirror is rigidly connected
to an underlying yoke. The yoke, in turn, is connected by two thin, mechanically
compliant torsion hinges to support posts that are attached to the underlying
substrate.

The address electrodes for the mirror and yoke are connected to the complementary
sides of the underlying SRAM cell.The yoke and mirror are connected to a bias
bus fabricated at the metal-3 layer. The bias bus interconnects the yoke and
mirrors of each pixel to a bond pad at the chip perimeter [36]. An off-chip
driver supplies the bias waveform necessary for proper digital operation (Section
2.4). The DMD mirrors are 16 um square and made of aluminum for maximum reflectivity.
They are arrayed on 17 um centers to form a matrix having a high fill factor
(~90%). The high fill factor produces high efficiency for light use at the pixel
level and a seamless (pixelation-free) projected image.

Electrostatic fields are developed between the mirror and its address electrode
and the yoke and its address electrode, creating an efficient electrostatic
torque. This torque works against the restoring torque of the hinges to produce
mirror and yoke rotation in the positive or negative direction. The mirror and
yoke rotate until the yoke comes to rest (or lands) against mechanical stops
that are at the same potential as the yoke. Because geometry determines the
rotation angle, as opposed to a balance of electrostatic torques employed in
earlier analog devices, the rotation angle is precisely determined.

The fabrication of the DMD superstructure begins with a completed CMOS memory
circuit. A thick oxide is deposited over metal-2 of the CMOS and then planarized
using a chemical mechanical polish (CMP) technique. The CMP step provides a
completely flat substrate for DMD superstructure fabrication, ensuring that
the projector's brightness uniformity and contrast ratio are not degraded.

Through the use of six photomask layers, the superstructure is formed with
layers of aluminum for the address electrode (metal-3), hinge, yoke and mirror
layers and hardened photo-resist for the sacrificial layers (spacer-1 and spacer-2)
that form the two air gaps. The aluminum is sputter-deposited and plasma-etched
using plasma-deposited SiO2 as the etch mask. Later in the packaging flow, the
sacrificial layers are plasma-ashed to form the air gaps.

The packaging flow begins with the wafers partially sawed along the chip scribe
lines to a depth that will allow the chips to be easily broken apart later.
The partially sawed and cleaned wafers then proceed to a plasma etcher that
is used to selectively strip the organic sacrificial layers from under the DMD
mirror, yoke, and hinges. Following this process, a thin lubrication layer is
deposited to prevent the landing tips of the yoke from adhering to the landing
pads during operation. Before separating the chips from one another, each chip
is tested for full electrical and optical functionality by a high-speed automated
wafer tester [55]. Finally, the chips are separated from the wafer, plasma-cleaned,
relubricated, and hermetically sealed in a package. Further manufacturing details
are contained in references [36, 43, 51, 52, 57].

The DMD pixel is inherently digital because of the way it is electronically
driven [5]. It is operated in an electrostatically bistable mode by the application
of a bias voltage to the mirror to minimize the address voltage requirements.
Thus, large rotation angles can be achieved with a conventional 5-volt CMOS
address circuit.

The organization of the DMD chip is shown in Figure 10.
Underlying each DMD mirror and mechanical superstructure cell is a six-transistor
SRAM. Multiple data inputs and demultiplexers (1:16) are provided to match
the
frequency capability of the on-chip CMOS with the required video data rates.
The pulsewidth modulation scheme for the DMD requires that the video field
time
be divided into binary time intervals or bit times. During each bit time, while
the mirrors of the array are modulating light, the underlying memory array
is
refreshed or updated for the next bit time. Once the memory array has been
updated, all the mirrors in the array are released simultaneously and allowed
to move
to their new address states.

This simultaneous update of all mirrors, when coupled with the PWM bit-splitting
algorithm described in Section 2.2, produces an inherently low-flicker display.
Flicker is the visual artifact that can be produced in CRTs as a result of brightness
decay with time of the phosphor.

Because CRTs are refreshed in an interlaced scan-line format, there is both
a line-to-line temporal phase shift in brightness as well as an overall decay
in brightness. DLP-based displays have inherently low flicker because all pixels
are updated at the same time (there is no line-to-line temporal phase shift)
and because the PWM bit-splitting algorithm produces short-duration light pulses
that are uniformly distributed throughout the video field time (no temporal
decay in brightness).

Proper operation
of the DMD is achieved by using the bias and address sequence shown in Figure
11 and detailed inTable 1.

The bias voltage has three functions. First, it produces a bistable condition
to minimize the address voltage requirement, as previously mentioned. In this
manner, large rotation angles can be achieved with conventional 5-volt CMOS.
Second,it electromechanically latches the mirrors so that they cannot respond
to changes in the address voltage until the mirrors are reset. The third function
of the bias is to reset the pixels so that they can reliably break free of surface
adhesive forces and begin to rotate to their new address states.

Although the metal surfaces of the superstructure are coated with a passivation
layer or lubrication layer, the remaining van der Waal or surface forces between
molecules require more than the hinge-restoring force to reliably reset the
mirrors. A reset voltage pulse applied to the mirror and yoke causes the spring
tips of the yoke (Figure 12) to flex.

As the spring tips unflex, they produce a reaction force that causes the yoke
landing tips to accelerate away from the landing pads,producing a reliable release
from the surface [52].

An improved hinge material that reduces metal creep that can occur under
high-duty-factor and high-temperature operating conditions.

Improved packaging
techniques that preserve the "lubricity" of the landing
surface over a wide range of environmental conditions.

A new architecture that incorporates spring tips at the landing tip of the
yoke. The result is greater operating margins as the yoke releases (resets)
from the underlying surface.

A particle reduction program that has dramatically reduced particle contamination
within the DMD package.

The DMD has
passed a series of tests to simulate actual DMD environmental operating conditions,
including thermal shock,temperature cycling, moisture resistance,
mechanical shock,vibration, and acceleration testing and has passed all of these
tests. In addition to these, other tests have been conducted to determine the
long-term result of repeated cycling of mirrors between the on and off states.
Mirror cycling tests look for hinge fatigue (broken hinges) and failure of the
mirrors to release because of increased adhesion (reset failure). To date, in
accelerated tests, a lifetime of more than 765 billion cycles has been demonstrated
(equivalent lifetime >76,000 hours) for a 10-bit/primary color, three-chip
projector configuration).

The first operation in the digital processor is progressive-scan conversion.
This conversion is required if the original source material is interlaced. An
interlaced format provides even lines of video during one video field time and
odd lines during the next field time. Progressive-scan conversion is the process
of creating (by an interpolation algorithm) new scan lines between the odd or
even lines of each video field.

Interlacing has been historically used in CRT-based systems to reduce the
video bandwidth requirements without producing objectionable flicker effects
created by the temporal decay in phosphor brightness. For progressively scanned
CRTs, interlacing is unnecessary because additional bandwidth is allocated so
that every line of the CRT is refreshed during each field time. Progressive
scanning that incorporates motion-adaptive algorithms helps to reduce interlace
scanning artifacts such as interline flicker, raster line visibility, and field
flicker. These are particularly noticeable in larger display formats.

The next operation in the digital processor is digital resampling (or scaling).
This operation resizes the video data to fit the DMD's pixel array, expands
letterbox video sources, and maintains a correct aspect ratio for the square
pixel DMD format. After the scaling operation, the video data is input to the
color space conversion block. If the video is not already in a red, green, blue
(R,G,B) format, it is converted from luminance and color difference encoding
(e.g., Y, CR , CB ) into R,G,B. Next, a degamma (inverse gamma) function is
performed because, unlike CRTs, DMDs are linear displays. The degamma operation
can produce low-light-level contouring effects, but these are minimized by using
an error diffusion technique.

Finally the R,G,B signal is input to the digital formatter. First,the scan-line
format data is converted into an R,G,B bit-plane format. The bit planes are
stored in a dual-synchronous DRAM (SDRAM) frame buffer for fast access of the
bit-plane data.The bit-plane data is then output to the DMDs in a PWM bit-splitting
sequence (Section 2.2). As explained in Section 2.4,the DMD chip has multiple
data inputs that allow it to match the frequency capability of the on-chip CMOS
with the required video data rates. The bit-plane data coming out of the frame
buffer is multiplexed 16:1 and fed to the multiple data inputs of each DMD.
The bit-plane data is then demultiplexed 1:16 and fed to the frame-memory underlying
the DMD pixel array.

3.2 Projection optics [44]

DLP optical systems have been designed in a variety of configurations distinguished
by the number of DMD chips (one,two, or three) in the system [44]. The one-chip
and two-chip systems rely on a rotating color disk to time-multiplex the colors.

The one-chip configuration is used for lower brightness applications and is
the most compact. Two-chip systems yield higher brightness performance but are
primarily intended to compensate for the color deficiencies resulting from spectrally
imbalanced lamps (e.g., the red deficiency in many metalhalide lamps). For the
highest brightness applications, three-chip systems are required.

A DLP optical system with three chips is shown in Figure 14.
Because the DMD is a simple array of reflective light switches, no polarizers
are required. Light from a metal halide or xenon lamp is collected by a condenser
lens. For proper operation of the DMD light switch, this light must be directed
at 20 degrees relative to the normal of the DMD chip (Figure 6).To accomplish
this in a method that eliminates mechanical interference between the illuminating
and projecting optics, a total internal reflection (TIR) prism is interposed
between the projection lens and the DMD color-splitting/-combining prisms.

The color-splitting/-combining prisms use dichroic interference filters deposited
on their surfaces to split the light by reflection and transmission into red,
green, and blue components.The red and blue prisms require an additional reflection
from a TIR surface of the prism in order to direct the light at the correct
angle to the red and blue DMDs. Light reflected from the on-state mirrors of
the three DMDs is directed back through the prisms and the color components
are recombined.The combined light then passes through the TIR prism and into
the projection lens because its angle has been reduced below the critical angle
for total internal reflection in the prism air gap.

A DLP three-chip prototype projection engine is shown inFigure 15.
It
projects 1100 lumens with a 500-watt xenon lamp. The size of the engine is 19.5
x 12.8 x 10 in. and it weighs 38 pounds. One of the DMD package assemblies with
thermoelectric cooler and fan is visible.

4.0 DISPLAY PERFORMANCE

4.1 Resolution

DLP projection systems have been demonstrated at a variety of resolutions
(and aspect ratios), VGA (640 x 480), SVGA (800 x 600) and SXGA (1280 x 1024).
A 16:9 aspect ratio high-definition (1920 x 1080) DLP projection system has
also been demonstrated [20, 27, 35]. Currently there are DLP-based products
on the market for business applications at SVGA resolution. Both professional
(high-brightness) and business products will be available at XGA resolution
by the end of 1997. SXGA products will follow in 1998.

The DMD family of chips uses a common pixel design having a 16 um mirror arrayed
with a 17 um pixel pitch. As the DMD resolution is increased, the pixel pitch
is held constant and the chip diagonal is allowed to increase. This approach
to the chip design has several advantages: (1) the high optical efficiency and
contrast ratio of the pixel is maintained at all resolutions, (2) pixel timing
is common to all designs and high address margins are maintained, and (3) the
chip diagonal increases with resolution, which improves the DMD system optical
efficiency (see Section 4.2).

4.2 Optical efficiency and brightness.

The optical efficiency of the DLP projection system is the product of the
efficiencies shown in Figure 17, namely the lamp/reflector, color filter/projection
lens, and pixel efficiencies.

The pixel efficiency is composed of the product of the efficiencies shown in
Figure 18, namely the fill factor, mirror "on" time, reflectivity, and
diffraction efficiency. For the DMD pixel design used today, the pixel efficiency
is 61% [44].

The color filter/projection lens efficiency depends on the dichroic filter
reflection and absorption losses and reflection losses in the projection lens
elements. For one-chip or two-chip DMD systems that use a rotating color disk,
there is an additional loss associated with the time-multiplexing of the colors.

The lamp/reflector efficiency depends on the amount of collected light that
can be used by the DMD: This is a function of the arc length of the lamp, the
reflector geometry, the area of the mirror array, and the cone angle ( f/#)
of the illumination and projection lens.

To understand the relationship of these parameters and their influence on
the lamp/reflector efficiency, it is useful to use the concept of etendue,
which is also known as "optical extent" or the "optical invariant." Etendue
is a measure of the area of the light distribution, convolved with the solid
angle of the light [60].

When a beam is modified by a well-corrected optical element,etendue is
preserved. For example, when a well-corrected lens focuses a collimated beam
to a spot, the area of the beam is reduced, but the divergence angle of the
beam increases and etendue is preserved. The lamp/reflector combination
has an etendue. The DMD/projection lens combination also has an etendue.
If the etendue of the DMD/projection lens is smaller than that of the
lamp/reflector, then the system is said to be etendue-limited. In this
case, not all of the collected light from the lamp/reflector can be used by
the DMD/projection lens. This is the case for all lamps but those with the shortest
arc lengths.

To maximize the lamp reflector efficiency, it is necessary to minimize the
etendue of the lamp in relation to that of the DMD/projection lens. The
etendue of the DMD/projection lens is given by E = pi A/4f 2 ,
where A is the area of the DMD and f is the f /# of the projection
lens. The f /# of the projection lens for the DMD is determined by the
mirror rotation angle of plus/minus 10 degrees. To adequately separate the on-state
light from the diffracted light produced by the mirror edges and substrate and
to maximize contrast ratio, an f /# no smaller than f /2.8 is
required. To preserve adequate contrast ratio in the resulting projected image,
the DMD projection lens is typically fixed at f /3.0. Thus the DMD/projection
lens etendue is determined solely by the area (resolution) of the DMD
and increases with resolution.

For a given resolution DMD, the lamp/reflector efficiency increases as
the lamp arc length (and etendue) decreases. For this reason,
short arc length lamps (<2 mm) are chosen for DLP applications. For a
given lamp etendue, the lamp/reflector efficiency increases as the
resolution (and etendue) of the dmd increases. This latter relationship
is shown graphically in Figure 19, where the modeled optical efficiency
(lumens/watt) of DLP three-chip projectors is shown plotted versus DMD resolution
for lamps of various powers. The SVGA resolution optical efficiencies (for the
500- and 900-watt lamps) are actual measurements from prototype projectors.
Also shown is the total luminous flux in lumens that can be delivered at SVGA
resolution.

For lower lamp power (lower brightness applications), the highest optical efficiencies
are achieved with metal halide(MH) lamps because of their high luminous efficacy.
However, as the lamp power is increased, the arc length of metalhalide lamps
must grow more rapidly than that of xenon (Xe)lamps to preserve lamp lifetime
(typically determined by the time for the brightness to diminish to 50% of the
stabilized brightness of a new lamp). Therefore, in DLP applications (for power
levels above ~300 watts), xenon lamps, because of their shorter arc, provide
better lamp/reflector collection efficiencies and higher overall system performance.

It should be noted that a large color gamut and good color balance (particularly
for flesh tones) are important in high-brightness applications such as digital
cinema. Although metalhalide lamps have a higher efficacy (lumens/watt) compared
to xenon, not all of the luminous output can be used if color balance is to
be preserved. Most metal halide lamps are typically characterized by having
strong green (mercury) and greenish yellow (sodium) emission lines. These lines
carry a significant portion of the lamp's luminous output. However,for proper
color balance, these lines must be attenuated, with a resulting drop in the
effective efficacy of the lamp.

Often it is difficult to compare projection system performance (in terms of
optical efficiency) of systems using xenon lamps versus those that use metal
halide lamps. Unless the color balance for these systems is adjusted to the
same specification (e.g., SMPTE-C RGB points and a D65 luminant color balance),
the luminous output of metal halide systems will naturally be overstated. Therefore,
the lamps of Figures 19and 20 have been color-balanced to achieve
a valid comparison of their performance.

In Figure 20, the modeled brightness (lumens) of DLP three-chip projectors
is plotted versus DMD resolution for lamps of various powers. For the 500-,
900-, and 1,500-watt lamps,the SVGA resolution brightness levels are actual
measurements from prototype projectors. Brightness levels up to 3000 lumens
at SVGA resolution have been demonstrated with short arc xenon lamps. The modeled
performance at HDTV resolutions with current lamps is projected to be 3600 lumens.With
further improvements in short-arc xenon lamp technology, Digital Projection
Ltd. (formerly Rank-Brimar) anticipates that brightness levels in excess of
10,000 lumens will be achieved in DLP brand products as resolution and format
approach HDTV standards.

4.3 Maximum brightness

At high luminous flux densities (lumens/cm 2 ), optical absorption creates
heating effects. Excessive temperature can cause degradation of performance
for both LCDs and DMDs. in the case of LCDs, excessive heating causes degradation
of the polarizers. Furthermore, without adequate cooling of the LCD panel, the
temperature of the LCD material can rise above its clearing temperature T c.
This renders the LCD material useless for polarization rotation and the display
fails. For transmissive AM-LCD panels, a heat sink cannot be attached to the
substrate, so forced air cooling must be relied upon. Larger transmissive panels
mitigate this problem. Currently,AM-LCD projectors having 3000-lumen outputs
use 5.8 x 5.8 in. panels.

Excessive temperatures
can also affect the long-term reliability of the DMD by accelerating hinge
deformation (metal creep) that can occur under high-duty-factor
operation of the mirror. Special hinge alloys have been developed to minimize
this deformation and guarantee reliable operation [52].High duty factors
occur
when the mirror is operated in one direction for a much greater part of the
time, on average, than in the other direction. For example, 95/5 duty factor
operation means that a mirror is 95% of the time at one rotation angle (e.g.,
-10 degrees) and 5% of the time at the other rotation angle (e.g., +10 degrees).
This situation would correspond to DMD operation with a video source having
a temporal average brightness of 5% (or 95%) of the peak brightness. Although
these extreme temporal averages are unlikely to occur for extended periods
of
time, 95/5 duty factor is chosen as a worst case reliability test condition
for hinge deformation. With current hinge metal alloys, long-term, reliable
DMD operation at the 95/5 duty factor is assured,provided the operating temperature
of the hinge is limited to <65 0 C.

For high-brightness applications, the mirrors can absorb enough energy to
raise the hinge temperature above 65_Cunless active cooling is applied to the
package. Because the dmd is reflective and built on a single-crystal silicon
(X-silicon) back plane, the absorbed heat can be efficiently extracted by connecting
a thermoelectric cooler (TEC) to the backside of the DMD package. In Figure
15, one of the DMD package assemblies with the thermoelectric cooler
is visible. The DMD package contains "thermal vi" to provide a low-thermal-impedance
path between the DMD chip and the TEC. A thermal model predicts that for a three-chip
SXGA projector producing 10,000 screen lumens, the hinge temperature can be
held to <65 0 C (with TEC cooling and an internal ambient air
temperature of 55 0 C) .

4.4 Contrast ratio

The inherent contrast ratio of the DMD is determined by measuring the ratio
of the light flux with all pixels turned on versus the flux with all pixels
turned off. The system contrast ratio is determined by measuring the light flux
ratio between bright and dark portions of a 4 x 4 checkerboard image according
to ANSI specifications. The checkerboard measurement takes into account light
scatter and reflections in the lens, which can degrade the inherent contrast
ratio of the DMD.

The full on/off contrast ratio determines the dark level for scenes having
a low average luminance level (e.g., outdoor night scenes) as well as the video
black level. The checker-board contrast ratio is a measure of the contrast for
objects in scenes containing a full range of luminance levels.

The inherent contrast ratio of the DMD is limited by light diffraction from
the mirror edges, from the underlying substrate,and from the mirror via (the
metallized hole in the middle of the mirror that acts as the mirror support
post, as shown in Figure 4). Recent architectural improvements to the
DMD pixels have led to improved contrast ratios (Table 2) [61].
Further improvements are expected.

4.5 Accuracy and stability

Current high-brightness projection displays for use in the audio/visual rental
and staging business and for private and corporate use have a number of limitations.
These include warm-up or stabilization time; setup time for convergence,color
balance, and gamma; and, finally, the stability of the image quality once the
system is operating. Maintaining stability over a wide range of environmental
conditions encountered in outdoor applications is particularly difficult.For
video wall applications or other applications requiring multiple side-by-side
projectors, the setup time to make all of the displays look identical is often
unacceptable. Even when great care has been taken in this procedure, lack of
stability makes periodic adjustments necessary.

DLP-based projection systems offer the potential of short setup time and stable,
adjustment-free images. Initial stabilization time is minimal. Convergence is
fixed by internal alignment of the three DMDs and is stable with time and independent
of throw distance. Color balance, uniformity, and gamma are digitally controlled
by pulsewidth modulation and are not affected by temperature. Brightness roll
off is stable (fixed by a light integrator) and can be made small to accommodate
video wall applications.

5.0 DLP BRAND PRODUCTS

Texas Instruments is teamed with numerous projection display manufacturers
spanning the business (conference room), consumer (home theater), and professional
(high-brightness) markets [62]. DLP brand products and prototypes serving all
three market segments have been demonstrated at numerous trade shows including
Cedia, Comdex, CES, Infocomm, EID,IFA, JES, Photokina, Photonics West, SID,
and Satis. Shipments of the first DLP brand business projectors began in March
1996. Soon the first consumer and professional products will be available on
the market.

Currently, Digital Projection Ltd., Electrohome, and Sony are developing high-brightness
DLP brand products with SVGA resolution and brightness levels ranging from 1100
lumens to 3000 lumens. Announcements of the first DLP brand professional products
is expected in the first quarter of 1997.

6.0 SUMMARY

DLP brand projection displays are well-suited to high-brightness and high-resolution
applications. The digital light switch is reflective and has a high fill factor
that results in high optical efficiency at the pixel level and low pixelation
effects in the projected image. The DMD family of chips uses a common pixel
design and a monolithic CMOS-like process. These factors, taken together, mean
that scaling to higher resolutions is straightforward, without loss of pixel
optical efficiency. At higher resolutions, the DLP brand projector becomes even
more efficient in its use of light because of higher lamp-coupling efficiency.
Because the DMD is a reflective technology,the DMD chip can be effectively cooled
through the chip substrate,thus facilitating the use of high-power projection
lamps without thermal degradation of the DMD.

DLP brand systems are all-digital (digital video in, digital light out) that
give accurate, stable reproduction of the original source material. DLP brand
projectors for the business (conference room) application are currently on the
market. Soon, the first consumer(home theater) products will be available. DLP
brand projection system prototypes for professional (high-brightness) applications
have been demonstrated at up to 3000-lumen brightness for SVGA resolution. Soon,
high-brightness SVGA products will be on the market, followed by XGA and SXGA
products. With anticipated improvements in short-arc xenon lamp technology,
it is expected that brightness levels in excess of 10,000 lumens should be achievable
in DLP brand products of the future, as resolution and format approach HDTV
standards.

7.0 ACKNOWLEDGEMENTS

The author wishes to acknowledge the numerous contributions of the Texas Instruments
Digital Imaging staff, with special thanks to the following individuals for
their many helpful suggestions during the preparation of this manuscript:Scott
Dewald, Mike Douglass, Jim Florence, Richard Gale, Richard Knipe, Vishal Markandey,
Greg Pettitt, Frank Poradish, and Peter van Kessel.

Special thanks
also go to the editors, Sara Kay Powers and Carolyn Banks; Larry Norton,
the illustrator; and the capable staff of CR&D Publishing
Services.